The numbers from his 10 years in the majors tell the story of a solid, steady infielder: 1,414 hits, .280 average, five teams, two All-Star elections, and two World Series rings, one with the Angels in ’02, the other with the Cardinals in ’06, when he also won World Series MVP honors after catching fire against the Tigers.

But the story of Eck—the 19th-round draft pick who became something of a baseball folk hero—was always told not with statistics but with adjectives from The Little Engine That Could: the plucky, determined overachiever who overcame his limitations. Eckstein laughs at the odd mix of idolatry and derision he inspired: “The back of my baseball card reads: OVERACHIEVING SMURF.”

Eckstein could have played a few more seasons. But there was one big reason to walk away after the 2010 season: to turn his attention to Her Universe, the company that he and his wife had founded the year before. Says Eckstein: “After all the years of her making my career a priority, it was time to make hers a real priority and really focus on growing the business.” Ashley had tabled her acting career—she had appeared in small roles in film and on TV—to move wherever David’s career took him. She had just begun working in voice-over on the popular animated series Star Wars: The Clone Wars when she noticed that all the series’ licensed apparel was designed for men.

“Playing for managers like Tony La Russa and Mike Scioscia, Eckstein says, taught him to foresee “1%” possibilities, a skill he calls “invaluable.”

Ashley hatched the idea of making sci-fi clothing and accessories exclusively for girls and women, which she saw as an untapped market. Close to half of all sci-fi fans are women, her research showed. “Eighty-five percent of all consumer purchases are made by women,” she says. “So why wouldn’t you give us something to buy?” The numbers suggest she was right. Her Universe expects 2014 sales of $5 million, according to Ashley, who says the company is profitable but declines to give specifics.

David and Ashley are unlikely entrepreneurs. Ashley’s career had been spent in front of cameras. David’s only commercial venture as a ballplayer did not exactly set the world on fire: In 2006, fresh off his series MVP award, he partnered with a cereal company on his own brand. A few months after Eckso’s went on sale, Eckstein was in a supermarket and saw boxes of his eponymous honey-nut toasted oats collecting dust in the discount aisle. When he brought a package to the checkout, the cashier—unable to connect the ballplayer on the box with the man he was ringing up—snickered, “When are these athletes going to learn that this stuff just doesn’t work?”

That’s not to say that selling sci-fi T-shirts, hats, and hoodies to fangirls has been easy. “Similar to David’s career—David vs. Goliath, where he kind of defied the odds—we really had to defy the doubters and defy the odds,” Ashley says. She says Lucasfilm rebuffed her twice times, until she partnered with a brand-management company. Since then, Her Universe’s licenses have expanded to include brands like Star Trek, Dr. Who, and the Walking Dead. The bulk of the Ecksteins’ time now is spent in meetings with potential partners as they seek new licenses. Says Ashley of her husband’s role: “He isn’t in on every call, but he’s in on all the financial decisions and comes to all the big meetings. If we have a new deal, he’s my closer.”

Ashley is a cult celebrity for her role in The Clone Wars. She is hounded by autograph seekers at sci-fi conventions, while David stays in the background, unrecognized—“I’m the guy holding the pink Caboodle,” he says. David tries to hold his own in this alien (for him) realm. At his first convention he repeatedly referred to a Star Wars light saber as a “life saver.” Says Ashley: “That was mortifying.” It’s getting easier for him. Eckstein recently talked to a man who couldn’t wait to play Dungeons & Dragons in his basement: “He kept referring to LARPing”—live-action role-playing—“and let’s just say the conversation was very quiet from my end. Now? I know all about it. It’s funny—LARPing in the basement may sound strange to a lot of people, but when you boil it down, it’s really the same thing as playing fantasy football. You tell someone who’s not a sports fan what you do, and they’re like, ‘You draft what, and you do what?’ The two worlds don’t really mix, and they would never admit that they’re alike in any way. But with the level of passion, they’re exactly the same.”

Eckstein isn’t yet spending his weekends LARPing, but he enjoys his new life. The strategy feeds his competitive side. “Playing for managers like Tony La Russa and Mike Scioscia, they break down games and they foresee a possibility that may be a 1% possibility, but they’ve already thought that through,” he says. Eckstein says those lessons are “invaluable” in his business life.

Eckstein’s true love, of course, remains baseball. He volunteer coaches at high schools and for Team USA baseball, and it seems inevitable that sooner or later he will return to baseball, either as a coach or as an executive. “It’s only a matter of time,” says Ashley.

For now, though, there’s a business to run. The Ecksteins still see their company as the little T-shirt company that could, trying to make it in the face of doubters (whether those doubters really exist or not). What fuels David these days is not unlike what motivated him all those years in the game. “The one thing you should not tell us is no—that we can’t do something,” he says. “Because we’ll do everything we can to prove you wrong. That’s the story of my life.”

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This story appears in the November 17, 2014 issue of Fortune.

Correction: A photo caption in an earlier version of this article incorrectly stated that the Ecksteins live in California. In fact, they live in Florida. In addition, the earlier version incorrectly stated that Her Universe was rebuffed three times by Lucasfilm; in fact, it was rebuffed twice. Fortune regrets the errors.